It is well-known that many materials show a tendency to decay over time and turn into extremely porous materials, before being completely destroyed, owing to the action by environmental chemical factors, or due to causes--also natural--decay or aging. Typical examples of materials which undergo such an action are many kinds and types of stones, such as bricks, marble, building materials, and so forth.
The problem of the preservation of these materials is of basic importance in those cases in which one wishes to preserve the concerned good, or building, with its original appearance, and a more or less integral replacement is not desired. This problem arises, e.g., in case of cultural, artistic, hystorical and archaeological goods, for which the witness has to be preserved for a time as long as possible.
It is well-known that the materials which are most subject to such effects of intense decay are the lithoidal monuments such as churches, palaces, buildings, and so forth, which, by being in continuous contact with the more or less polluted atmospheres of towns and/or of the surrounding environment, are exposed to continuous attack by such chemical agents as sulfur oxides or nitrogen oxides which, in the presence of atmospheric humidity, exert an action which modifies to a substantial extent the nature of the materials they were built from. These chemical attacks, combined with the natural changes of temperature, causes changes in color, or the formation of crusts, scales, films, crevices, and so forth, with a consequent progressive and unarrested loss of in material cohesion.
In this case, the intervention for preservation requires a preliminary cleaning and a subsequent protecting treatment. These steps protect the various elements or parts, in such a way as to protect the good, or its most meaningful details, such as decorations, reliefs, and so forth, from the aggressive agents.
In order to obtain such protection, it is known to coat or impregnate the lithoidal good or material with film-forming polymeric substances such as acryl, vinyl, silicone resins, and so forth, which show good adhesive properties, good resistance to the atmospheric agents and a fairly good resistance to aging caused by the action of light or of other atmospheric agents. The main drawback shown by these polymeric substances is that they form a surface film which, although is very thin, is not permeable to air, vapors and other gases. Such an impermeable coating leads to the formation of accumulations of vapors in the material, to the concentration of salts and to a full range of processes which cause even evident and irreparable damage to the treated material. In fact, it is well-known that the "breathing" of a lithoidal material, whether of stone, brick, marble or the like, is of basic importance for a correct and durable preservation thereof.
The purpose of the instant invention is to provide a product for protecting lithoidal materials which, besides being colorless and having good properties of adhesion and very good characteristics of resistance to light, moisture, water and to atmospheric agents, does not substantially modify the permeability to gases and vapors, and furthermore shows no film-forming properties, as well as characteristics of reversibility.
The present Applicant has found now that polyurethanes with hydroxy functionality, containing moieties of perfluoropolyether in their chain, are products showing the above specified characteristics.